In the drilling and production of oil and gas wells, it is often necessary to obtain inner surface information concerning conditions within a borehole. For example, tools and other objects may become lodged in the borehole during the drilling of a well. Such objects must be retrieved before drilling can continue.
In the operation and/or periodic maintenance of producing or injection wells, it is frequently necessary to obtain information about the construction and/or operating condition of production equipment located downhole. For example, detection of the onset of corrosion damage to well tubing or casing within a borehole enables the application of anti-corrosive treatments to the well. Early treatment of corrosive well conditions prevents the highly expensive and dangerous replacement of corrosion damaged well production components.
Other maintenance operations in a production well environment, such as replacement of various flow control valves or the inspection of the location and condition of casing perforations, make it highly desirable for an operator located at the surface to obtain accurate, real-time information about downhole conditions.
In fact, new regulations require operators of oilfields to perform a visual inspection of their safety/barrier valves after certain operations to verify cleanness to secure a further safe operation. These are often referred to as BlowOut Preventers (BOP) which are a large, specialized valves or similar mechanical device, usually installed redundantly in stacks, used to seal, control and monitor oil and gas wells, and intended to prevent tubing (e.g. drill pipe and well casing), tools and drilling fluid from being blown out of the wellbore (also known as bore hole, the hole leading to the reservoir) when a blowout threatens.
Preferable, the above-mentioned inspection should be executed with an image sensor while the BOP is in position on the seabed. The main challenge preventing such an operation is the contaminated fluid at the point of interest. Quite often, expensive drilling rigs unsuccessfully try to displace the contaminated water with clean water to achieve images of subsea equipment. The water remains too contaminated to achieve quality images, and the consequence is that the BOP needs to be pulled to surface to be visually inspected onboard the rig and rerun thereafter. This operation involves several heavy lifts and is very time demanding; several days of lost operation.
Other tubulars may need inspection. This is the case of risers, large tubulars connecting Oil & Gas exploration or production platforms or ships to subsea installations.
Various techniques have been proposed for obtaining at the surface information about the conditions within a borehole, well, pipe or other tubular constructions filled with contaminated fluid with and image sensor/camera. One example is disclosed in U.S. Pat. No. 4,938,060, to Halliburton (ex-OTIS), inv Sizer et al. It includes a method of injecting coiled tubing having an inspection sensor into a wellbore to a selected location, injecting an optically transparent or acoustically homogenous fluid into the wellbore through the coiled tubing to form a slug of such fluid around the sensor, and transmitting signals from the sensor representative of well conditions to the surface. The method may be practiced to inspect only the region around the sensor at a selected depth in the well or may be continuously practiced to examine the length of the wellbore by producing the well and retrieving the coiled tubing and sensor at a controlled rate synchronized with the rate of well production.
The solution proposed in the document U.S. Pat. No. 4,938,060 only provides point wise transparency between the image sensor and the object of interest. The effect is also time limited since the optical transparent fluid is ejected into the contaminated fluid, so that the situation of opacity around the image sensor will shortly return.
The document US2007127780 relates to visual inspection of an interior surface of a borehole. A housing adapted to be lowered in the borehole supports a portable camera for generating images of a portion of the interior surface of the borehole and a light source for illuminating an area adjacent the camera. The ability of the arrangement shown in the document to inspect the interior of a hollow member is limited.
It is therefore a need in inspection systems of tubular constructions filled with contaminated fluid for an arrangement improving signal transfer between the image sensor and the surface to be inspected.